Conventionally, in a differential transmission system for transmitting a synchronized signal through a pair of communication lines, a magnetic flux generated by a normal mode current is canceled. Thus, a normal mode noise is reduced, so that a noise in the communication lines is mainly a common mode noise generated between the lines and a ground. Here, the common mode noise is determined by a common mode impedance. In a slave node connecting to a master node through a pair of communication lines for communicating with a differential transmission system, the common mode impedance is defined by grounding to a conductive ground plate. Thus, the common mode noise is reduced. This is disclosed in, for example, JP-A-2002-354053, JP-A-2003-18224 and JP-A-2004-96351. For example, in JP-A-2004-96351, a terminal circuit in a differential transmission line provides a center tap terminal circuit for adjusting a differential mode and a common mode to the differential transmission line. The center tap terminal circuit includes a resister and a capacitor, and is connected in series with an input terminal of a receiving side IC.
A passenger protection device control system operates a passenger protection device such as an air bag system and a seatbelt pre-tensioner when a vehicle collides. In the control system, an ECU as a master node is coupled with various sensors and a starting device as a slave node through a pair of communication lines, respectively. The sensors are, for example, a collision sensor and a passenger-detecting sensor. The starting device is an air bag squib as an igniting device for operating the airbag.
In the conventional passenger protection device control system, it is necessary to maintain reliability of communication between the master node and the slave node even when a part of the communication lines is broken by the vehicle collision. Thus, each sensor functions as a non-grounded slave node, which is not grounded to a body earth such as a vehicle body. Accordingly, the common mode impedance of each sensor is not defined so that the common mode noise is reduced.
Thus, it is required for a circuit system functioning as a non-grounded slave node in a differential transmission system to reduce a common mode noise.